Membrane based separation processes such as reverse osmosis, pervaporation and perstraction are conventional (i.e., well-known). The pervaporation process is a technique of separation of liquid mixtures. A low-pressure vacuum is maintained on one side of the membrane media to provide a low energy approach to vaporizing liquid materials. The vaporization temperature of these liquid materials under vacuum conditions is lower than the temperature needed at elevated pressures. The liquid mixtures to be separated are conducted onto an upstream side of a membrane, which is essentially impervious to some of the liquid components but will permit selective passage of other components in a controlled manner. The downstream side of the membrane is usually exposed to a vacuum and the liquid components passing through the membrane can be removed in the vapor phase and condensed in a condenser.
In a pervaporation process, a desired feed component, e.g., the aromatic component, of a mixed liquid feed is preferentially dissolved into the membrane film. For membranes selective for the desired component, the desired component is preferentially adsorbed by the membrane. A membrane is exposed at one side to a stream of the mixture and a vacuum is applied to the membrane at the opposite side so that the liquid compound adsorbed migrates through the membrane via the well-known solution-diffusion mechanism. Accordingly, the desired component passes through the membrane and is removed as vapor from its downstream side, thereby providing room for the additional adsorption of the desired component on the upstream side of the membrane. A concentration gradient driving force is therefore established to selectively pass the desired components through the membrane from the upstream side to the downstream side.
Various membranes have been used in the prior art. For instance, U.S. Pat. No. 4,861,628, and U.S. Pat. No. 5,030,355 describe separating aromatics from non-aromatics such as naphtha, heavy catalytic naphtha (HCN), etc., by for example pervaporation using a suspension-coated membrane. The membrane is formed by depositing a polymer on a porous support layer, which is a fine dispersion or suspension and not a solid mass.
Among the conventional apparatus used with the pervaporation techniques are membranes used with spiral-wound and plate frames. For instance, in U.S. Pat. No. 3,398,091 covering a membrane separation using a cell comprised of a stack of basic units between a pair of end plates is disclosed. The semi-permeable membranes are held by spacers and a support. Heat transfer fluid is conducted via an inlet, conduit system, heating compartments and across heat transfer sheets.
Prior art pervaporation processes have used discrete equipment steps to achieve the desired separation. Interconnection of these equipment devices for large flow rate applications is expensive. Also, the prior art spiral-wound elements are expensive and difficult to manufacture for high temperature services. The majority of commercial spiral-wound element designs are limited to the 100-120 degree temperature range. To achieve separations of gasolines, naphthas, diesel fuels and higher boiling hydrocarbons usually higher temperatures are needed in excess of 120° C. There therefore is a need for an apparatus and process to apply pervaporation of fluids to hydrocarbon materials, especially those used as transportation fuels, to achieve separation of specific molecular types in an economical and efficient fashion.